With a conventional radar system, a pulsed signal is transmitted and the time taken for the pulse to travel to the object and back allows the range of the object to be determined. In a passive radar system, there is no dedicated transmitter. Instead, the receiver uses third-party transmitters and measures the time difference of arrival (TDOA) between the signal arriving directly from the transmitter and the signal arriving via reflection from the object, allowing the bi-static range of the object to be determined. In addition to bi-static range, passive radar can also measure the bi-static Doppler shift of the echo and also its direction of arrival allowing the location, heading and speed of the object to be calculated. In some cases, multiple transmitters and receivers are used to make several independent measurements of bi-static range, Doppler and bearing and hence significantly improve the final track accuracy.
The Passive Coherent Location (PCL) system is bi-static radar, which measures the elliptical distance and the Doppler frequency shift. It works with continuous wave (CW) transmitters of opportunity, meaning that it uses electromagnetic radiation, primarily assigned for another purpose, for example, radio or television terrestrial broadcasts. It is necessary to detect at least two (in an ideal case three or more) direct signals from transmitters for a proper determination of a target position.
As of December 2006, there are several PCL systems in various stages of development or deployment, including:                Silent Sentry is a Lockheed Martin (USA) PCL system that uses FM radio transmissions. Two different antenna variants are believed to be available providing an antenna that provides 360° azimuth coverage from 4 different beams (an Adcock array), and a variant that provides 100° azimuth coverage from six different beams (linear array). It has a range of up to around 100 nautical miles depending on the variant employed and a number of receive nodes at different locations can be combined to provide increased coverage. See, http://www.dtic.mil/ndia/jaws/sentry.pdf, incorporated herein by reference.        Celldar is a British system developed jointly by Roke Manor and BAE Systems. The system is a PCL sensor that can exploit GSM signals, currently in the 900 MHz band, but may also be able to use the 900 MHz and 1800 MHz bands simultaneously in the future. It is believed that Celldar is a low level/surface surveillance system designed to achieve good coverage below 10,000 ft and can track targets in 2D over a 100° sector at ranges of up to around 60 km. See, http://www.roke.co.uk/skills/radar/, incorporated herein by reference.        CORA is a German PCL sensor, developed by FGAN (Die Forschungsgesellschaft für Angewandte Naturwissenschaften e.V.), that exploits Digital Video Broadcast-Terrestrial (DVB-T) and Digital Audio Broadcast (DAB) transmissions.        Cristal is a PCL sensor developed by Thales that exploits FM radio transmissions to track targets. In addition to Cristal, it is believed that Thales has a prototype PCL system that uses analog TV or DAB transmissions.        One of the PCL systems developed by ERA, formerly Rannoch Corporation, (www.rannoch.com) uses FM radio transmissions.        
Each of these systems rely on continuous wave (CW) communications whether or not the CW signal is modulated to provide analog or digital information, as the techniques basically rely on the comparison of delayed versions of the source (i.e., the reflections) with the original CW signal. For example, analog signals include conventional FM radio or television, while digitally encoded signals include new television formats for audio, video, and telecommunications (e.g., DAB, DVB, and GSM).
U.S. Pat. No. 7,155,240, entitled “Method of Determining the Position of a Target Using Transmitters of Opportunity,” (Atkinson et al.), and incorporated herein by reference, describes a technique for non-reliance on line of sight with a digital source signal such as GSM. That technique claims a method of determining the position of a target using components in a wireless communication system in which pre-stored codes are included in transmissions of communications signals as part of a communication protocol, comprising the steps of: a) providing a transmitter which transmits a communications signal; b) providing a plurality of receivers, in communication with each other, which receive communications signals reflected from the target, the receivers being disposed at locations which are separate from the transmitter and separate from each other, and being time or phase synchronized; c) determining a time of arrival information of the received communications signal at each receiver by continuously correlating the code in the received communications signal with the pre-stored codes in the receiver; and d) using information pertaining to the location of each receiver, together with the information obtained from step c), to determine the target position.
In essence, the technique described by Atkinson et al uses a priori information relating to digital encoded signals where the receiver essentially identifies embedded data formats such as headers, lead-ins, or other recognizable formats. The technique appears to have been developed with digital communications in mind, and is not described for older analog transmission such as conventional television, FM radio, or other analog signals. Essentially, Atkinson's patent relies on unique or known characteristics contained within the raw digital data encoding of the transmitted signal for time or phase referencing.